Myotonic Dystrophy (DM) is the most common form of muscular dystrophy in adults and includes two clinically similar diseases although originating from distinct genetic mutations. DM1 (Steinert's disease) corresponds to the majority of the cases of DM and stems from an expansion of the CTG trinucleotide repeat (typically more than 50 units) in the 3′-untranslated region (UTR) of the DMPK gene whereas DM2 originates from big expansions of the CCTG tetranucleotide in the first intron of the CNBP gene. Both are rare diseases, with a global combined prevalence estimated at 12.5 per 100000, although it has been suggested that it could be three times higher (Suominen, T. et al. Population frequency of myotonic dystrophy: higher than expected frequency of myotonic dystrophy type 2 (DM2) mutation in Finland. Eur J Hum Genet. 19(7):776-82, 2011). DM1 is a multisystemic disease that affects primarily to the skeletal muscle (miotonia, muscle weakness and degeneration), the heart and the central nervous system (Gomes-Pereira, M. et al. Myotonic dystrophy mouse models: towards rational therapy development. Trends Mol Med. 17(9):506-17 (2011)).
Disease severity and age of onset are critically linked to expansion size. Whereas in classic adult onset the approximate number of repeats is 50 to <500, adolescent onset is 500 to <1000, childhood onset is 1000 to <1.500, and congenital onset is equal or superior to 1500. Due to these differences, currently, there is not clear if the pathogenesis of congenital DM1 (CDM) is similar to adult DM1. In fact, unknown factors account for the unique features of CDM as there is no animal model of DM1 that produces a typical CDM phenotype (Campbell, et al. Congenital Myotonic Dystrophy J. Neurol. Neurophysiol. S7-001, XP055186364, 2012)
Cells carrying few CTG repeats show a functional equilibrium between two antagonistic splicing regulators: muscleblind-like 1 (MBNL1) and CUGBP/Elav-like family member 1 (CELF1). The balance between MBNL1 and CELF1 controls the establishment of adult splicing profiles for a subset of developmentally regulated transcripts. The CTG repeat expansion expressed in a DM1 cell forms an imperfect double-stranded stem loop structure that has two main pathogenic consequences: MBNL1 sequestration by RNA foci and protein kinase C (PKC)-mediated and AKT-mediated CELF1 hyperphosphorylation, stabilisation and subcellular redistribution. As a result of MBNL1 depletion and CELF1 upregulation, the balance between these two splicing regulators is disturbed and the alternative splicing of a series of developmentally regulated transcripts reverts to a foetal pattern. The abnormal expression of splicing isoforms in adult skeletal muscle, heart and brain is likely to contribute to the DM1 disease symptoms. Whereas MBNL1 seems critical in skeletal and cardiac phenotypes, sequestration of the MBNL2 paralog has been suggested to originate brain symptoms and MBNL3 may inhibit muscle differentiation. In addition to defective alternative splicing regulation, protein translation deficits, Repeat-Associated non-ATG translation (RAN translation) and bidirectional transcription of the DMPK gene, altered expression of microRNAs, gene transcription deficiencies, and RNA interference, among others, have been shown to contribute to molecular alterations typical of DM in different cell and animal models of the disease.
Under expression of MBNL or over expression of CELF1, or a combination of both situations as exists in human DM1 manifests in a multisystem illness due to modifications of RNA splicing.
DM1 is a truly multisystem disorder, primarily affecting skeletal muscles (myotonia, muscle weakness and degeneration), the heart and the central nervous system (CNS) (Gomes-Pereira, M. et al. Myotonic dystrophy mouse models: towards rational therapy development. Trends Mol Med. 17(9):506-17 (2011)). The great variability of DM1 symptoms and age of onset results in three main clinical forms of the disease: late-onset, classical adult onset and congenital DM1. DM1 can occur in patients of any age whereas DM2 typically presents in adults. Both DM1 and DM2 patients can also be considered as a person who carries the expanded CTG or CCTG mutation, respectively.
Myotonia (delayed muscle relaxation after initial contraction) and progressive wasting of distal muscles are prominent features of DM1 in skeletal muscle. The more severe congenital form of DM1 is characterized by general muscle hypotonia and respiratory distress at birth, as well as delayed motor development.
A large portion of patients suffer from cardiac conduction blocks, detected by electrocardiogram (ECG), and cardiac histological abnormalities. Progressive cardiopathy can result in complete atrioventricular block or ventricular arrhythmias and subsequent sudden death in almost 30% of DM1 patients. Particularly, problems related to the cardiopulmonary system accounts for 70% deaths due to DM1.
CNS manifestations are highly debilitating and support the view that DM1 is also a brain disorder. DM1 neuropsychological dysfunction is accompanied by histological abnormalities, as well as brain structural changes and altered metabolism.
The impact of DM1 further affects a variety of tissues and results in presenile cataracts, abnormal glucose tolerance and hyperinsulinism, gastrointestinal dysfunction and testicular atrophy. See Gomes-Pereira et al. Trends in Molecular Medicine, 2011, 17(9), 506-516
According to the Myotonic Dystrophy Foundation (http://www.myotonic.org), the symptomatology of the myotonic dystrophy can be expressed by the following events                1. Skeletal muscle: myotonia, progressive wasting and weakness, pain, general hypotonia in congenital DM1        2. Heart: cardiac condition defects, prolonged PR intervals, first degree atrioventricular block, arrhythmias        3. Central nervous system: hypersomnolence, cognitive impairment, executive dysfunction, visual-spacial memory deficits, neuropsychological changes, mental retardation in congenital DM1        4. Smooth muscle: gastrointestinal complications, swallowing issues, abdominal pain, abnormal motility, malabsorption, constipation/diarrhea, anal incontinence        5. Respiratory system: breathing problems in newborns, frequent lung infections, aspiration of food or fluids into airways, inability to breathe in enough oxygen, sleep apnea        6. Hormonal system: hyperinsulinism (diabetes), male prefrontal balding        7. Immune system: hypogammaglobulinemia.        8. Vision: premature subcapsular iridescent and muticoloured cataracts, damage to the retina, drooping eyelids (ptosis)        9. Reproductive system: testicular atrophy, small testes, reduced fertility, low sperm count, low testosterone, higher risk of miscarriage and stillbirth, early menopause, problems with pregnancy and delivery, newborn complications, more severe with each generation (“anticipation”)        10. Skin: Higher risk of benign skin tumor (pilomatrixoma)        
Current therapeutic strategies are based on candidate drugs that bind to CUG or CCUG repeats, thereby releasing MBNL proteins to regulate splicing, or other processes, of its pre-mRNA targets. An increase in free MBNL1 protein has been shown to reduce the severity of symptoms in animal models of DM1, while in contrast, a decrease of free MBNL1 protein is observed with longer expansions, which are correlated with more severe disease. The pathogenic role of the MBNL1 gene is further substantiated by the fact that MBNL1 gene variants modify DM1 severity (Vincent Huin et al. MBNL1 gene variants as modifiers of disease severity in myotonic dystrophy type 1. J Neurol (2013) 260:998-1003). Similarly, MBNL proteins have been shown to get sequestered in DM2, thus, strategies aimed at increasing steady state levels of these proteins have the potential to become treatments for DM2. Then, the consensus strategy to reverse splicing abnormalities observed in DM1 and DM2, among other molecular alterations, is based in a reduction in nuclear foci and the concomitant reduction in nuclear sequestration of MBNL proteins. Warf and colleagues (Warf, M., Nakamori, M., Matthys, C. Thornton, C. and Berglund, J. (2009) Pentamidine reverses the splicing defects associated with myotonic dystrophy. Proceedings of the National Academy of Sciences of the United States of America, 106, 18551-18556. Coonrod, L., et al. (2013) Reducing levels of toxic RNA with small molecules. ACS Chem Biol. 8(11):2528-37) demonstrated that pentamidine facilitates a partial rescue of DM1 pathology.
Since then, some other therapies have been disclosed, such as antisense oligonucleotide compounds (see, EP 2 560 001A2, US 2011/0269665 A1), pentamidine analogues (see, A., Haley, M., Thornton, C. et al. (2013) Reducing levels of toxic RNA with small molecules. ACS chemical biology, 8, 2528-2537; Parkesh, R., Childs-Disney, J., Nakamori, M., Kumar, A., Wang, E., Wang, T., Hoskins, J., Tran, T., Housman, D., Thornton, C. et al. (2012) Design of a Bioactive Small Molecule That Targets the Myotonic Dystrophy Type 1 RNA via an RNA Motif-Ligand Database and Chemical Similarity Searching. Journal of the American Chemical Society, 134, 4731-4742), peptides (see, EP 2 554 180 A1, Gareiss, P., Sobczak, K., McNaughton, B., Palde, P., Thornton, C. and Miller, B. (2008) Dynamic combinatorial selection of molecules capable of inhibiting the (CUG) repeat RNA-MBNL1 interaction in vitro: discovery of lead compounds targeting myotonic dystrophy (DM1). Journal of the American Chemical Society, 130, 16254-16261; Garcia-Lopez, A., Llamusi, B., Orzaez, M., Perez-Paya, E. and Artero, R. (2011) In vivo discovery of a peptide that prevents CUG-RNA hairpin formation and reverses RNA toxicity in myotonic dystrophy models. Proceedings of the National Academy of Sciences of the United States of America, 108, 11866-11871), or chemical drug candidates (see, U.S. Pat. No. 8,754,084 B2, EP 2 742 974 A1, U.S. Pat. No. 8,741,572 B1, US 2014/0051709 A1, Jahromi, A. H., Nguyen, L., Fu, Y., Miller, K. A., Baranger, A. M. and Zimmerman, S. C. (2013) A novel CUG(exp). MBNL1 inhibitor with therapeutic potential for myotonic dystrophy type 1. ACS chemical biology, 8, 1037-1043; Wong, C. H., Nguyen, L., Peh, J., Luu, L. M., Sanchez, J. S., Richardson, S. L., Tuccinardi, T., Tsoi, H., Chan, W. Y., Chan, H. Y. et al. (2014) Targeting Toxic RNAs that Cause Myotonic Dystrophy Type 1 (DM1) with a Bisamidinium Inhibitor. Journal of the American Chemical Society, 136, 6355-6361; Childs-Disney, J., Stepniak-Konieczna, E., Tran, T., Yildirim, I., Park, H., Chen, C., Hoskins, J., Southall, N., Marugan, J., Patnaik, S. et al. (2013) Induction and reversal of myotonic dystrophy type 1 pre-mRNA splicing defects by small molecules. Nature communications, 4, 2044; Hoskins, J. W., Ofori, L. O., Chen, C. Z., Kumar, A., Sobczak, K., Nakamori, M., Southall, N., Patnaik, S., Marugan, J. J., Zheng, W. et al. (2014) Lomofungin and dilomofungin: inhibitors of MBNL1-CUG RNA binding with distinct cellular effects. Nucleic acids research, 42, 6591-6602).
The generic cardiovascular drug mexiletine, initially developed to treat heart rhythm abnormalities, has been reported to hold some potential for treating muscle stiffness and other symptoms of non-dystrophic myotonias (NDMs). Mexiletine-induced sodium channel blockade reduced myotonia in small studies (Stantland, J M, et al. Mexiletine for symptoms and signs of myotonia in nondystrophic myotonia: a randomized controlled trial, JAMA. 2012, 308(13), 1357-1365).
Therefore, although DM1 was first described in 1909, there is presently no cure or specific treatment for myotonic dystrophy. All the treatments applied are palliative and contribute to control the development of a subset of the overall plethora of symptoms, and the clinical focus is on managing the complications of the disease, but in no case for treating the disease in a definitive manner.
Therefore, there is an existing need for compounds and compositions which can treat myotonic dystrophy, in particular myotonic dystrophy type 1 and type 2, in a more effective way.
The present inventors have surprisingly found that the compounds described herein cover this existing need. In addition, the compounds disclosed herein can be advantageously found directly in natural products or can be readily synthesized in just a few chemical steps.
Some of the compounds described herein are known by those skilled in the art as derivatives of xanthine (known collectively as xanthines or methylxanthines). Methylated xanthines (methylxanthines), include caffeine, aminophylline, IBMX, paraxanthine, pentoxifylline, theobromine, and theophylline. This group of alkaloids that mainly act as adenosine receptor blockers, is commonly used as mild stimulants and as bronchodilators, notably in the treatment of asthma symptoms. In contrast to other more potent stimulants like sympathomimetic amines, xanthines mainly act to oppose the actions of the sleepiness-inducing adenosine, and increase alertness in the central nervous system. They also stimulate the respiratory centre, and are used for treatment of infantile apnea and the apnea of prematurity (AOP). AOP is a common problem affecting premature infants, likely secondary to an immature respiratory system. Methylxanthine therapy is a mainstay of treatment of central apnea by stimulating the central nervous system and respiratory muscle function. The most common cause of apnea specially in newborns is prematurity, to whom the most common drugs used to treat apnea are the methylxanthines (Henderson-Smart et al. Prophylactic methylxanthine for prevention of apnoea in preterm infants. Cochrane Database Syst Rev. 2010 Dec. 8; (12):CD000432. doi: 10.1002/14651858.CD000432.pub2), theophylline (Scanlon et al. Caffeine or theophylline for neonatal apnoea? Arch Dis Child. 1992 April; 67(4 Spec No): 425-428) and aminophylline (Larsen et al. Aminophylline versus caffeine citrate for apnea and bradycardia prophylaxis in premature neonates. Acta Paediatr. 1995 April; 84(4):360-4).
The inventors have surprisingly found that xanthines are able to increase the amount of free MBNL1 in human DM1 myoblasts. An increase in free MBNL1 protein has been shown to reduce the severity of symptoms in animal models of DM1, while in contrast, a decrease of free MBNL1 protein is observed with longer expansions, which are correlated with more severe disease. The pathogenic role of the MBNL1 gene is further substantiated by the fact that MBNL1 gene variants modify DM1 severity (Vincent Huin et al. MBNL1 gene variants as modifiers of disease severity in myotonic dystrophy type 1. J Neurol (2013) 260:998-1003). Similarly, MBNL proteins have been shown to get sequestered in DM2, thus, strategies aimed at increasing steady state levels of these proteins have the potential to become treatments for DM2
No reference is found in the literature for the direct use of these compounds for the purposes disclosed herein.